1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a transmitting/receiving apparatus and method for ensuring high data rate and high reliability in a multiple-input multiple-output (MIMO) mobile communication system.
2. Description of the Related Art
Adaptive Modulation and Coding (AMC) is actively studied to increase channel capacity in wireless networks. The AMC scheme adapts a modulation scheme and a code rate according to the current signal quality or channel state, while keeping transmit power constant during transmission of one frame, to increase data rate. For example, assume a nearby mobile station (MS) has a small error probability in receiving signals from a base station (BS). The nearby MS uses a high-order modulation scheme such as 16-ary quadrature amplitude modulation (16QAM) in which four bits form one signal, and a high code rate such as ¾. As a remote MS receives signals with a high error probability from the BS, the remote MS uses a low-order modulation scheme and a low code rate to receive signals without errors.
In AMC, the transmission scheme used is decided with respect to a plurality of code rates and modulation schemes depending on what region a signal-to-noise ratio (SNR) measured at a receiver belongs to in an SNR versus throughput graph (or curve). However, the SNR versus throughput graph is derived from a particular fading channel model, which implies that the SNR versus throughput graph may not be viable in a changed fading environment. Accordingly, a need exists for a new method of selecting a transmission scheme.
Typically, multiple antennas are used for two purposes: one is to increase data reliability and the other is to increase data rate. To achieve a higher reliability and a higher data rate, the data reliability increasing scheme is adopted in a bad channel state and the data rate increasing scheme is adopted in a good channel state. Obviously, the adaptive use of a multiple antenna scheme according to a channel state leads to the increase of data rate and data reliability.
In this context, combining the traditional AMC scheme and a multiple transmit/receive antenna scheme has emerged as a significant issue for future communication systems. Satisfactory data rate and reliability can be achieved by optimizing a modulation scheme, a code rate, and an antenna transmission scheme according to a channel state, taking into account both the AMC and the multiple antenna scheme which serve the same purpose.
Technologies of exploiting multiple antennas have been studied to build a high-speed, reliable communication system that maximizes data transmission rate and minimizes an error rate with limited radio resources. These technologies are called space-time processing.
Space-time processing was developed to cope with problems encountered in a wireless environment, such as signal loss and unexpected channel state degradation. With their introduction in the 1960's, beam-forming algorithms are now an area of active study for the purpose of increasing cell capacities and antenna gains on the forward link and the reverse link. As is well known, the Space Time Coding (STC) proposed by Tarokh, et al. in 1997 achieves a diversity effect proportional to the product of the number of transmit and receive antennas.
Considerable efforts have also been made to exploit multiple antennas for increasing data transmission rate as well as for improving reception performance as described above. One of the most prominent data rate increasing methods is spatial multiplexing (SM). The SM is a scheme of transmitting different information data through different transmit antennas. Telta, et al.'s study results reveal that the SM brings as much an increase in capacity as the number of transmit antennas, compared to single-input single-output (SISO). The capacity increase is significant to a high-speed data transmission system.
The STC aims to reduce the error rate of transmission data and the SM is used to maximize the data rate of transmission data. One method that serves both purposes is 2-layered SM. For example, in a system where four transmit antennas act as two transmit antennas by grouping, two STBC blocks corresponding to the two transmit antennas can be connected. In this case, a higher data rate is achieved than in an STBC scheme using four antennas, and a lower error rate than in an SM method using four antennas.
As described above, there exist a variety of MIMO (Multiple-Input Multiple-Output) schemes according to the numbers of transmit antennas and receive antennas in the communication system. Therefore, system capacity depends on which one is selected for data transmission/reception among the various antenna combinations.
Accordingly, a need exists for developing a method of determining an antenna transmission scheme according to a fading channel environment, while still applying the traditional AMC scheme, in a MIMO system.